The invention relates to Medium Access Control (MAC) protocols in CSMA networks.
Data transmission systems or networks use some type of medium access control protocol to control access to the physical medium, e.g., an AC power line or Ethernet cable, if the medium is a shared medium. That shared medium access mechanism may be polling, Time Division Multiple Access (TDMA), token passing, Carrier Sense Multiple Access (CSMA), or some other shared access protocol. Polling uses a centrally assigned master station to periodically poll other (slave) stations, giving those other nodes explicit permission to transmit on the medium. In the TDMA protocol, a network master broadcasts a frame synchronization signal before each round of messages to synchronize clocks of all stations and, after synchronization occurs, each station transmits during a uniquely allocated time slice. In the token passing scheme, access to the transmission medium is determined by possession of a special data unit called a token, which is passed from station to station. In the CSMA protocol, all transmissions are broadcast on the medium and stations listen to the medium to determine when the medium is idle before transmitting. In CSMA with collision avoidance (CSMA/CA), each station listens to the medium while each transmission is in progress and, after the transmission ends, waits a specified interval (or interframe gap) followed by an additional delay of one or more transmission (or contention resolution) slots based on a selected slot number before transmitting.
There are variations on one or more of these protocols related to prioritization, which may be necessary to ensure efficient station-to-station dialog or Quality of Service (QoS) requirements. For example, with CSMA/CA, the first slot may be reserved for a response by the station that just received a message, or, alternatively, one or more of the slots may be reserved for transmissions (or stations) of a particular priority class.
In some particularly noisy network environments, such as powerline network environments, the CSMA protocol operation (as well as peer-to-peer communication) may be adversely affected by distances between stations and/or channel conditions. Additionally, the CSMA protocol operation may be susceptible to interference (e.g., collisions) caused by overlapping networks, that is, networks that are not intended to communicate with one another but can in fact hear each other""s transmissions, as well as by a hidden station (or node). The hidden station is a station in a network that, because of its location, may hear only half of a communication exchange between other stations in the same or neighboring networks. Under such circumstances and conditions, the CSMA scheme may not function effectively to maintain proper network synchronization and orderly media access arbitration. Another consequence is that the network may be unable to ensure strict adherence to QoS guarantees.
In one aspect of the invention, in a network of stations interconnected by a transmission medium, operating a station according to a medium access protocol includes preparing a frame to include at least one frame control field for conveying frame control information to be used by substantially all of the stations for medium access contention, the frame control information including contention control information, and causing the frame to be transmitted onto the transmission medium.
Embodiments of the invention may include one or more of the following features.
The frame can include a payload and the at least one frame control field can further include a first frame control field preceding the payload. The at least one frame control field can further include a second frame control field following the payload. The contention control information can include a contention control flag for indicating to the other stations whether a next channel access in the next contention period is to be contention-based or contention-free. The contention control flag can be a bit which, if set, indicates the contention-free access.
The at least one frame control field can include a type field. The type field can indicate that the frame control field is a first frame control field in a start delimiter preceding a payload. The first frame control field of the start delimiter can include a payload length field for specifying the length of the payload. The first frame control field of the start delimiter can further include a channel map index for specifying a channel map, the channel map including channel map information to be used to demodulate the payload. The type field can indicate that the frame control field is a second frame control field in an end delimiter following a payload and that the second frame control field includes a channel access priority field specifying a channel access priority level assigned to the frame.
The frame can include a payload that includes a MAC management information field for conveying MAC management information to at least one of the other stations.
The type field can indicate that the at least one frame control field is in a response frame, and that the at least one frame control field includes a channel access priority. The channel access priority and the contention control information can be copied from the first frame control field in the start delimiter of a data frame transmission to which the response frame responds. The channel access priority and the contention control information can be copied from the second frame control field in the end delimiter of a data frame transmission to which the response frame responds. The channel access priority can be copied from a segment control field of a body of a frame in a data transmission to which the response frame responds. The at least one frame control field of the response frame can further include a received frame check sequence field for specifying a portion of a cyclic redundancy check that appeared in a data frame transmission to which the response frame responds and is to be used by the data frame transmission to determine the validity of the response frame.
The first frame control field in the start delimiter can further include a channel access priority.
When the frame includes a payload, the at least one frame control field and the payload can be encoded differently so that the at least one frame control field is capable of being heard by all of the stations while the payload is not capable of being heard by all of the stations.
In other aspect of the invention, in a network of stations interconnected by a transmission medium, operating a station according to a medium access protocol includes transmitting a frame onto the transmission medium in a frame transmission, the frame including frame control information that is placed within the frame in such a way as to be capable of being detected by the other stations in the network at more than one point during the frame transmission.
In yet another aspect of the invention, in a network of stations interconnected by a transmission medium, operating a station according to a medium access protocol includes transmitting the information as a data frame onto the transmission medium in a data frame transmission, the data frame causing transmission of a response onto the medium as a response frame transmission, the data frame and the response including frame control information for use by the other stations in determining when a next medium access contention period will occur and whether the other stations can contend during the next contention period, and capable of being detected by the other stations at different points during the data frame and response transmissions.
In still yet another aspect of the invention, in a network of stations interconnected by a transmission medium, operating a station according to a medium access protocol includes detecting in a frame transmission on the medium frame control information, the frame control information being placed within the frame in such a way as to be capable of being detected at more than one point during the reception of the frame transmission.
Embodiments of the invention may include one or more of the following features.
The frame control information can be used to determine when a next medium access contention may occur. Using the frame control information can include setting virtual carrier sense information based upon the receipt of the frame control information to indicate the timing of the next medium access contention relative to the end of the frame transmission when the frame control information is valid and a queued frame has a channel access priority greater than any identified by the frame control information or, otherwise, to indicate network idle time. The virtual carrier sense information can include a virtual carrier sense timer and associated flag, the associated flag being set to a one when the virtual carrier sense time indicates the timing of the next medium access contention. The frame control information can indicate the position of the frame control information within the frame and whether a response is expected, and the timing of the virtual carrier sense timer can be set to a value based on the indication of the position of the frame control information and whether a response is expected.
Among the advantages of the present invention are the following. The frame control field data (e.g., contention control indicator, channel access priority) and their positioning within a data frame transmission and an associated response (if expected) provide for a more robust communication of channel access information to a network""s stations, all of which rely on such information for proper synchronization and efficient access, thus reducing the risk of interference and collisions. For example, a station that hears only one half of the traffic on the transmission medium, perhaps an acknowledgement but not the data transmission to which the acknowledgment responds, is able to detect the contention control status in the frame control field of one of the delimiters (in this case, the frame control field in the response) and will not interfere with contention-free traffic by contending for access at an inappropriate time.
Other features and advantages of the invention will be apparent from the following detailed description and from the claims.